The present invention relates to devices and systems for holding a spinal fixation element. More particularly, the invention provides closure systems including at least an outer locking nut having an inner nut driving configuration for attaching a spinal fixation rod to a vertebral coupling element such as a pedicle screw or a hook.
The use of spinal fixation instrumentation to align and/or fix a desired relationship between adjacent vertebral bodies is well established. Such instrumentation typically includes a spinal fixation element, such as a relatively rigid fixation rod, that is coupled to adjacent vertebrae by attaching the element to pedicle screws which have been inserted into the patient""s vertebrae or to spinal hooks which can be placed into a vertebral arch for coupling to the vertebral bodies. Once installed, the spinal fixation instrumentation holds the vertebrae in a desired spatial relationship, either until desired healing or spinal fusion has taken place, or for some longer period of time.
One example of a rod based spinal fixation system is provided in U.S. Pat. No. 5,005,562, issued Apr. 9, 1991 to Cotrel (which is hereby incorporated by reference). This system includes pedicle screws and spinal hook vertebral coupling elements (both screws and hooks) having integral U-shaped bodies that extend outward from the vertebrae to which they are attached. A spinal fixation rod is shaped as desired and fitted into the xe2x80x9cUxe2x80x9d of U-shaped bodies of adjacent vertebrae. The inner surfaces of the U-shaped body are threaded to accept a set screw, and rod is fixed to the vertebral coupling elements by threading a set screw into each of the U-shaped bodies to lock in the rod.
U.S. Pat. No. 5,545,165, issued Aug. 13, 1996 to Biedermann et al. (and incorporated herein by reference), illustrates an improvement in closure systems for fixing a rod to vertebral coupling elements over those provided by Cotrel. The Biedermann et al. system also uses pedicle screws and spinal hooks having U-shaped bodies that extend outward from the vertebrae to which they are attached. The U-shaped bodies of the Biedermann et al. system are threaded on both the inside and the outside. The rod is therefore locked in by both an inner set screw and an outer lock nut. In the illustrated embodiments, the inner set screw is adapted to be driven on its threads using a hex-shaped driver element, and the outer locking nut is provided with hex-shaped flat outer surfaces suitable for engagement with a wrench or similar driving tool.
U.S. Pat. No. 5,443,467, issued Aug. 22, 1995 to Biedermann et al. (and incorporated herein by reference) illustrates the use of an inner set screw and an outer lock nut to lock a rod into a U-shaped body in a polyaxial screw system. In this system, a pedicle screw having a spherical head is captured within a separate U-shaped receiver body. The angle of the screw with respect to the body can be changed until a head-locking element is tightened to lock the angle of the screw head within the receiver body. According to Biedermann et al., this combination of an inner set screw and an outer locking nut provides an advantage in that the force acting on the rod can be independently adjusted by either the inner set screw or the outer locking nutxe2x80x94a particularly useful advantage where the rod being fastened is curved and an exact fastening might only be possible by independent adjustment of the two closure elements. In addition, when tightened, the inner set screw and the outer locking nut tend to lock each other in their tightened positions.
While the closure systems of the Biedermann et al. patents have been quite successful, the illustrated embodiments necessarily involve the use of an externally engaging driving element such as a wrench. For many applications, this driving configuration poses no problems. In some applications, however, especially in lumbar spine applications where the curvature of the spine causes the U-shaped bodies to extend toward each other, the preferred placement of the spinal coupling elements may result in the U-shaped members being so proximate to each other that attachment of the outer locking nuts having external hex surfaces can be challenging for the surgeon.
The present invention provides closure devices and systems having smaller profiles that include internal driving elements to allow for spinal coupling elements to be placed closer to each other than had previously been possible. In a first aspect, the invention includes a low profile spinal fixation element closure device. The closure device includes opposed proximal and distal ends, a generally cylindrical, low profile outer surface, an inner surface defining an inner bore extending through the closure device, and a spinal fixation element closure surface provided on the distal end. An engagement element is formed on the inner surface adjacent the distal end for engaging a spinal fixation element receiving body so as to close the spinal fixation element to the spinal fixation element receiving body upon tightening of the engagement element. A driving element is formed at the proximal end within a periphery defined by the low profile outer surface and is configured to receive a driving tool to tighten the engagement element.
In a further aspect of the invention, a closure system for fixing a spinal fixation element to a spinal fixation element receiving body is provided having a low profile outer locking nut for engaging an outer surface of the spinal fixation element receiving body and an inner set screw for engaging an inner surface of a spinal fixation element. The outer locking nut includes opposed proximal and distal ends, a generally cylindrical, low profile outer surface, an inner surface defining an inner bore, and a spinal fixation element closure surface provided on the distal end. An engagement element is formed on the inner surface adjacent the distal end for engaging an outer portion of the spinal fixation element receiving body so as to close the spinal fixation element to the spinal fixation element receiving body upon tightening of the engagement element. A driving element is formed at the proximal end within a periphery defined by the low profile outer surface and is configured to receive a driving tool to tighten the engagement element.
The inner set screw includes a body having an outer surface, a proximal end and a distal end. A spinal fixation element closure surface is provided on the distal end, a fastening element is formed on the proximal end, and an engagement element is formed on the outer surface. The engagement element engages an inner portion of the spinal fixation element receiving body so as to close the spinal fixation element to the spinal fixation element receiving body upon tightening of the engagement element.
The outer locking nut and inner set screw are configured so that when the outer locking nut is engaged to an outer portion of the spinal fixation element receiving body and the inner set screw is engaged to an inner portion of the spinal fixation element receiving body, the fastening element of the inner set screw is accessible through the inner bore of the outer locking nut so that the outer locking nut and the inner set screw are independently tightenable with respect to the spinal fixation element receiving body.
A still further aspect of the invention provides a system for coupling a spinal fixation element to a patient""s spine having a spinal fixation element receiving body, a low profile outer locking nut, and an inner set screw. The spinal fixation element receiving body includes a vertebral coupling element disposed on a first end of the body and a spinal fixation element receiving opening formed on a second end of the body. The body further includes an outer portion adapted to engage the outer locking nut and an inner portion adapted to receive and engage the inner set screw. The outer locking nut and inner set screw can be similar to those described above.
In specific embodiments of the invention, the engagement element of the outer locking nut can include threads formed on the inner surface adjacent to the distal end and the driving element of the outer locking nut can formed in the inner surface of the central bore, for example by including a female driving element including a plurality of angled sides. In addition, the inner bore of the outer locking nut can have a minimum diameter that is sufficiently large to permit a driving tool to extend into the bore to drive the inner set screw when the outer locking nut and the inner set screw are assembled to the spinal fixation element receiving body, and can further be sufficiently large to permit the inner set screw to pass through the inner bore of the outer locking nut.
An outer locking nut driving tool configured to engage the female driving element to drive and thereby tighten the outer locking nut to the spinal fixation element receiving body can be providing. This driving tool may also be hollow, defining an inner bore having an inner diameter sufficiently large to permit an inner set screw driving tool to pass through the outer lock nut driving tool so that the outer locking nut and the inner set screw can be tightened at the same time.